High intensity discharge (HID) lamps such as metal halide (MH) and high pressure sodium (HPS) lamps have increasingly gained acceptance over incandescent and fluorescent lamps for commercial and industrial applications. HID lamps are more efficient and more cost effective than incandescent and fluorescent lamps for illuminating large open spaces such as construction sites, stadiums, parking lots, warehouses, and so on, as well as for illumination along roadways. An HID lamp comprises at least an arc-tube containing two electrodes, chemical compounds and a fill gas. The fill gas can comprise one or more gases. To initiate operation of the lamp, the fill gas is ionized to facilitate the conduction of electricity between the electrodes.
HID lamps can be difficult to start. An HID lamp such as a conventional HPS lamp uses a 2500 to 4000 volt pulse at least once per half-cycle and at selected times during the cycle in order to start, as set forth in a number of standards such as ANSI C78.1350 on HPS lamps, for example. An ignitor is used to provide the necessary pulses to start the conventional HID lamp. If the lamp is extinguished after lamp operation has elevated lamp temperature, the lamp cannot be restarted until after the lamp cools down and the fill gas can be ionized again. For many types of HID lamps, this lamp cooling period can be between approximately 40 seconds and 2.5 minutes, which can be considered unacceptable in situations where, for example, emergency lighting is desired.
A number of HID lamp-types are being manufactured using high pressure xenon as the fill gas. The fill gas is used because it is easier to ionize than the mercury and sodium in the lamp when the operational temperature of the lamp is low. When used with suitable ignition circuitry, these types of lamps require less time to restart after the lamp is extinguished and the temperature of the lamp has been elevated by lamp operation. As the xenon gas in the lamp ionizes, the relative concentration of the xenon gas begins to decrease (i.e., the xenon gas pressure decreases) while the operating temperature of the lamp and the relative concentration of the sodium vapor increase. Consequently, as the concentration of sodium vapor increases, it becomes easier to ionize the sodium and therefore to illuminate the lamp. To initiate the ionization process under both cold and hot lamp conditions, however, an ignitor and a hot restrike ignitor are required.
Both conventional ignitors and hot restrike ignitors initiate ionization by generating a series of high frequency, high voltage pulses (i.e., typically greater than 3000 volts) across the base of the lamp. Both types of ignitors generate pulses at or near the peak of an input sine wave to generate sufficient energy to ionize the fill gas inside, for example, an HPS lamp. The major difference between a standard ignitor and a hot restrike ignitor is that a restart ignitor produces a pulse which is higher in voltage and contains significantly more energy than a pulse generated by a standard ignitor (e.g., on the order of 7000 volts). This energy is typically stored in one or more capacitors. The pulses are generated when the capacitor(s) discharge through a transformer. Accordingly, a hot restrike ignitor can start a high pressure xenon-type HPS lamp or other lamp having a hot restrike capability, even though the concentration of xenon gas is relatively low as compared with the relative concentration of hot, de-ionized sodium vapor. A standard ignitor must, on the other hand, wait until the HID lamp sufficiently cools and the voltage and energy required to re-ionize the fill gas decreases to a sufficiently low level.
A number of circuits have been developed to hot restrike HID lamps. These hot restrike ignitors generally include resistors, pulse transformers and other components, in addition to a conventional ballast. These devices can reduce system efficiencies and substantially increase system cost.
In commonly-assigned U.S. Pat. Nos. 5,047,694 and 5,321,338, circuits for starting, operating and hot restriking an HPS lamp are described. U.S. Pat. Nos. 5,047,694 and 5,321,338 are incorporated herein by reference. An exemplary circuit is depicted in FIG. 1. In the circuit shown in FIG. 1, terminals 10 and 11 are connected to an alternating current (AC) source 13 which is typically a 240 line voltage source. A power factor correcting capacitor 12 is connected between the terminals 10 and 11. An inductive ballast indicated generally at 14 is typically a lag-type ballast and has terminals connected to the terminal 10 and to one terminal of an HPS lamp 16, respectively. The other terminal of the HPS lamp 16 is connected to terminal 11 such that the ballast 14 and the lamp 16 are in series across the AC source terminals 10 and 11. The ballast 14 is a tapped ballast having winding portions 18 and 19 and a tap 20 provided at the junction therebetween.
A semiconductor switch 63 such as a silicon-controlled rectifier (SCR) or the like is connected so that one end of its switchable conductive path is connected to the end of the first portion 18 of the ballast. The other end of the conductive path of the SCR 63 is connected to the tap 20 via a storage capacitor 62. A number of sidacs 64 or other breakdown devices are connected between the gate and the anode of the SCR 63. A current-limiting resistor 65 is provided in series with the sidacs 64. If the voltage on the capacitor 62 increases to a level which reaches or exceeds the threshold voltage of the breakdown devices 64, the sidacs 64 become conductive, placing the SCR 63 in a conductive state. Accordingly, the capacitor 62 discharges through the portion 18 of the ballast. Because the winding portions 18 and 19 of the ballast are electromagnetically coupled, the portion 18 of the ballast operates as the primary of a transformer in that a voltage is induced in the winding portion 19. The high voltage generated in the winding portion 19 of the ballast 14 is imposed on the lamp 16. The relationship of the winding portions 18 and 19 is selected to create a voltage using the SCR 63 and the sidacs 64 which is sufficiently high to start a lamp 16.
A charging circuit for the capacitor 62 is connected between the tap 20 and the terminal 11 at the other side of the AC power source 13. This charging circuit includes a two diodes 69, a pumping capacitor 66 and two radio frequency chokes 67 connected in series between the tap 20 and the terminal 11. Two diodes 68 are connected between the capacitor 66 and a thermostatic switch 70 and are poled in the opposite direction from the diodes 69. The charging circuit provides for the controlled, step-charging of the storage capacitor 62. The switch 70 provides for the automatic turn-off of the charging circuit for the capacitor 62 after a selected period of time has elapsed.
The starter circuit 60 depicted in FIG. 1 further comprises a more conventional HPS lamp starting aid comprising a capacitor 76 connected in series with a resistor 78, a choke 80 and a sidac 82 or similar breakdown device, which is connected between the resistor-capacitor junction and the tap 20. The charge on the capacitor 76 increases through the resistor 78 and the choke 80 until the breakdown voltage of the sidac 82 is reached. The capacitor 76 then discharges through the portion 18 of the ballast 14 to produce a starting voltage pulse.
The conventional HPS lamp starting aid of the starting circuit 60 is operable when the lamp is cold. To hot restrike the lamp, however, the circuit described above for charging and discharging the capacitor 62 is needed. The starting circuit 60 is configured for operation with lamps that respond to hot restriking following a momentary power interruption. Conventional HPS lamps, which requiring cooling before they can be started after a power interruption, do not respond well to the type of ignition pulse generated by the circuit depicted in FIG. 1. On the other hand, a lamp such as a 600 watt high pressure xenon HPS lamp does not start reliably using a conventional ignitor. The circuit depicted in FIG. 1 is disadvantageous because it does not reliably and predictably generate the high voltage pulses required to start the lamp. Additional high voltage starting pulses are generated by the components 76, 78 and 82 of the conventional starting circuit, which appear in an attenuated form across the semiconductor switches 64. These additional pulses cause premature triggering of the SCR 63. This early activation of the SCR 63 results in a reduced ignitor pulse being generated by the capacitor 62 since its energy is being discharged at a lower voltage level.
Accordingly, a need exists for an ignitor which can predictably and reliably start a conventional HID lamp, or start or restart an HID lamp having a hot restrike capability such as a high pressure xenon HPS lamp. Such an ignitor is also useful since an increasing number of HID lamps such as HPS lamps are being manufactured with high pressures of xenon gas in order to improve system performance and reduce the amount of mercury that is used in the lamps. As stated previously, consumers may consider the time elapsing between when a conventional HID lamp is extinguished and its next ignition to be too long, and may opt to use a high pressure xenon HPS lamp or similar lamp that does not need to cool prior to restart for more effective emergency lighting. Thus, a need exists for an ignitor which allows consumers to choose the type of HID lamp that they wish to use and which will operate with either a conventional HID lamp or a lamp capable of hot restrike such as a high pressure xenon HPS lamp.